1. Field of the Invention
This invention relates to an optical recording medium, particularly to an optical recording medium for recording layer comprising at least one trimethine-cyanine dye with specific functional group and an additive, which can enhance the electric properties of DVD recordable system.
2. Description of the Related Art
An optical recording medium capable of once recording by a laser ray has been known. In the structure of the CD-R recording medium, a recording layer comprising an organic dye, a reflective layer and a protective layer are laminated on a transparent substrate by order. The recording information on the disc is performed by irradiating a laser at a wavelength of 780 nm to 830 nm to cause exothermic deformation of the recording layer. The information is usually reproduced by irradiating with the same laser ray, and the reflecting difference between the recorded area and unrecorded area is detected for the recording layer.
It is known that a laser ray with a shorter wavelength has a reduced light spot. Therefore, studies have being made to develop the optical disc capable of recording and reproducing by a laser beam of a wavelength of 620 nm to 690 nm to increase the recording density. Recordable digital video (DVDxe2x88x92R), which is capable of recording and reading in high density, is propagated as new media of the next generation. Since the wavelength zone for performing the recording and reproducing in DVDxe2x88x92R differs from that of the CD-R, the criteria for selecting the recording material to be employed for the CD-R cannot be applied to the DVDxe2x88x92R. Hence, new selection criteria are required to be established from a viewpoint peculiar to the DVDxe2x88x92R.
There is also a new type disc named DVD+R. Although there are some differences at the disc specification between DVDxe2x88x92R and DVD+R, but the writing wavelength of laser beam domain in the same range, that is from 645 nm to 670 nm, so the DVD+R system still needs a recording material with high sensitivity at the recording wavelength.
Trimethine-based cyanine dye is a popular material for the DVDxe2x88x92R recording layer. U.S. Pat. No. 5,976,658, and U.S. Pat. No. 6,291,045 described the use of trimethine-based cyanine dye having symmetry or asymmetry indolenine structure for DVDxe2x88x92R. However, the dye material should be selected carefully, while the writing wavelength of laser beam of DVDxe2x88x92R machines is changed from 635 nm to 660 nm (or further). Because the wavelength of laser power is getting longer, the dye absorbance of film state for 660 nm should be increased a little more. The dye, which is suitable for writing wavelength of 635 nm laser beam, is not performed well while the writing wavelength of laser beam is 660 nm because low sensitivity at the region might cost higher writing power to record information or fail in record.
Said in U.S. Pat. No. 6,338,888, increase the sensitivity of recording could be reached by increasing the absorption of organic layer at the wavelength of writing laser. According to the opinion, we declare a new composition with specific additives, which could enhance the recording sensitivity.
The first object of the present invention is to provide an optical recording medium having a recording material which contains at least one trimethine-based cyanine dye with the formula (I): 
and an additive, which has a little absorption at the wavelength of write/read laser beam, such as any one of the following formulas (II) to (IX): 
The second object of the present invention is to provide an optical recording medium with improved write/read characteristics at the writing wavelength of 660 nm or further.
According to the invention, the asymmetric trimethine-cyanine dye with electron-withdrawing substituted group (EWG) used in the recording layer is any one represented by the general formula (I).
In formula (I), xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d, different from each other, represent any one of the following general formulas (1), (2), (3), and (4): 
One of the substituted group: (D1)p, (D2)q, (D3)r, (D4)s, which is substituted on the ring xe2x80x9cAxe2x80x9d or ring xe2x80x9cBxe2x80x9d, should be an electron-withdrawing substituted group, such as cyano and nitro group.
xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d, may be the same from each other, represent any one of the following general formulas (2), (3), and (4).
(D2)q, (D3)r, (D4)s may be any one of the substituted or unsubstitued alkyl, carboxyl, alkoxycarbonyl, alkylcarboxyl, alkoxyl, alklhydroxyl, aralkyl, alkenyl, alkylamide, alkylamino, alkylsufonamide, alkylcarbamoyl, alkylsulfamoyl, hydroxyl, halogen atom, hydrogen atom, alkylalkoxyl, alkylhalide, alkylsulfonyl, alkylcarboxyl or alkylsulfonyl group which are bonded to a metallic ion or alkyl, phenyl, benzyl, alkylphenyl or phenoxyalkyl group.
xe2x80x9cpxe2x80x9d, xe2x80x9cqxe2x80x9d, xe2x80x9crxe2x80x9d, xe2x80x9csxe2x80x9d represent the number of subsistent groups, which is an integer of 1 or more.
xe2x80x9cCxe2x80x9d and xe2x80x9cDxe2x80x9d may be represent any one of the general formulas (1), (2), (3), and (4).
xe2x80x9cR1xe2x80x9d and xe2x80x9cR2xe2x80x9d may be the same or different from each other and are individually substituted or unsubstitued alkyl, phenyl, benzyl, alkylphenyl, allyl or phenoxyalkyl group.
xe2x80x9cZxe2x80x9d is hydrogen atom, halogen atom or alkyl group
xe2x80x9cXxe2x80x9d is an anion selected from the group consisting of Fxe2x88x92, Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, ClO4xe2x88x92, BF4xe2x88x92, PF6xe2x88x92, SbF6xe2x88x92, OTsxe2x88x92, OMsxe2x88x92, SCNxe2x88x92, alkylsulfonat akylcarboxylate.
xe2x80x9cYxe2x80x9d could be an atom such as O, S, Se, N, or substituted or unsubstitued alkyl group, such as C(Me)2.
xe2x80x9cR3xe2x80x9d to xe2x80x9cR15xe2x80x9d may be the same or different from each other and are individually substituted or unsubstitued alkyl, phenyl, benzyl, alkylphenyl or phenoxyalkyl group.
xe2x80x9cEDGxe2x80x9d represents any electron-donating substituted group such as amino, alkylamino and methoxy group.
xe2x80x9cMxe2x80x9d is a center atom, it could be a hydrogen atomxe2x96xa1 a metal atom or metal oxide, such as Cu, Pd, Ni, Fe, Co, VO.
xe2x80x9cnxe2x80x9d is an integral, such as 1,2.
Further, the method of preparing dye solution according to this invention is provided, and described as follows.
(1) A cyanine dye represented by general formula (I) is dissolved in an organic solvent, such as fluorinated alcohol, diaceton alcohol, methylethyl ketone, methanol, toluene, cyclohexanone, acetylacetone, and dioxane. The concentration of the dye solution is 0.5% to 20% by weight.
(2) A dye mixture, which contains cyanine dye represented by general formula (I) mixed with an additive of formula (II) to (IX), that is less than 50% by weight, is dissolved in an organic solvent such as fluorinated alcohol, diaceton alcohol, methylethyl ketone, methanol, toluene, cyclohexanone, acetylacetone and dioxane. The concentration of the dye solution is 0.5% to 20% by weight.
Further, the manufacture of the optical recording medium according to this invention is provided, and performed as follows.
(A) The material for the substrate may be glass, epoxy resin, methacrylate resin, polycarbonate, polyester resin, polyvinyl chloride resin, or polyolefin resin. The substrate may be provided with the tracking grooves or pits.
(B) The dye solution is provided on a substrate by means of a spin-coating method.
(C) After spin-coating, the substrate with dye film is baked at the temperature of 90xc2x0 C. for 1 hr to 3 hr.
(D) The optical recording medium may further include a reflective layer in addition to the recording layer. The reflective layer, single or multiple films with high reflectivity, can be formed by the vapor-deposition or sputtering a metal, such as Gold, Aluminum, Silver, Cupper, Platinum, or alloy.
(E) The reflective layer may be provided with a protective layer. It is formed for the purpose of protecting or improving the optical recording medium, and can be formed by coating a solution of a radiation cure type resin on a given surface, followed by radiation-curing the coater layer.
(F) The optical recording medium may be a single-side recording medium obtained by laminating a single-side substrate and a non-grooved substrate with an adhesion. The single-side optical recording medium has been prepared by process (A) to (E).
(G) The optical recording medium may be a double-side recording medium obtained by laminating two single-side optical recording medium with an adhesion. The single-side optical recording medium has been prepared by process (A) to (E).
Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.